Impact of ciprofloxacin in the human-flora-associated (HFA) rat model: Comparison with the HFA mouse model Agne `s Perrin-Guyomard a, * , Jean-Michel Poul a , Michel Laurentie a , Pascal Sanders a , A. Hayde ´e Ferna ´ndez b , Mary Bartholomew b a Agence Franc ¸aise de Se ´curite ´ Sanitaire des Aliments, Laboratoire d’e ´tudes et de Recherches sur les Me ´dicaments Ve ´te ´rinaires et les De ´ sinfectants, BP 90203, 35302 Fouge `res cedex, France b United States Food and Drug Administration, Center for Veterinary Medicine, 7500 Standish Place, Rockville, MD 20855, USA Received 23 August 2005 Available online 24 March 2006 Abstract The ecological impact of different doses of ciprofloxacin was investigated in an experimental germ-free rat model into which human fecal flora was inoculated. Animals received oral doses (gavage) of 0, 0.25, 2.5, and 25 mg/kg body weight (bw) of ciprofloxacin once daily for 5 weeks. All doses of ciprofloxacin significantly reduced aerobic populations. Elimination of Enterobacteriaceae and reduction of bifodibacteria were noticed in the group treated with 25 mg/kg of the antibiotic. The rest of the intestinal flora was not affected. These effects were reversible after the treatment ended. The percentage of resistant enterococci increased in rats treated with 2.5 and 25 mg/kg; however, this increase was not statistically significant. There was a significant (P < 0.05) emergence of ciprofloxacin-resistant Bacteroides fragilis group with 25 mg/kg bw, which is equivalent to a human therapeutic dosage of the antibiotic. The MIC values and the percentage of resistance remained elevated 2 weeks after the end of treatment in this anaerobic population. Although sub-populations of enterococci and Enterobacteriaceae showed decreased susceptibility after ciprofloxacin administration, resistance was not evident. The ability of an exogenous strain of Salmonella to colonize the intestine of animals treated with 25 mg/kg of ciprofloxacin confirmed that the drug dis- rupted the colonization barrier effect of the indigenous flora at the high dose level tested. No changes in the metabolic parameters occurred during the antibiotic treatment. The results obtained in the HFA rat model were similar to those obtained in our previous study using the HFA mice model where ciprofloxacin at 0.125, 1.25, and 12.5 mg/kg bw induced a decrease of enterococci and Enterobacte- riaceae populations. The high dose of ciprofloxacin also induced a decrease in bifidobacteria counts, an increase in levels of resistant B. fragilis group and a significant (P < 0.05) disruption of the colonization resistance of the barrier flora in HFA mice. Published by Elsevier Inc. Keywords: Human-flora-associated rodent model; Intestinal flora; Ciprofloxacin; Drug residues 1. Introduction The use of antimicrobials in food-producing animals may result in antibiotic residues in edible products which might produce adverse effects on the human intestinal flora after ingestion. European and United States guidelines for veterinary drug registration recommend that, in the estab- lishment of the acceptable daily intake (ADI) of a drug, the microbiological hazards from antimicrobial residues must take into account the potentially harmful effects of the res- idues on the human intestinal flora (CVM, 2000; JECFA, 2002). In vitro or in vivo approaches are used by animal health industries, contract laboratories, and regulatory authorities to assess the effect of ingested antimicrobial res- idues on human intestinal flora. The ADIs for drug resi- dues have been determined using minimum inhibitory concentrations (MICs) values of a drug tested against pure strains isolated from intestinal flora; however, ADIs deter- mined by this method do not take into account the interac- tion of intestinal microorganisms, the conditions in the gastrointestinal tract, or the metabolism of the host. www.elsevier.com/locate/yrtph Regulatory Toxicology and Pharmacology 45 (2006) 66–78 0273-2300/$ - see front matter Published by Elsevier Inc. doi:10.1016/j.yrtph.2006.02.002 * Corresponding author. Fax: +33 2 99 94 78 80. E-mail address: a.perrin-guyomard@afssa.fr (A. Perrin-Guyomard).